The present invention relates to a method and apparatus for automatically adjusting the sensing threshold in an automatic implantable cardioverter/defibrillator with pacing capability.
In the field of cardiac rhythm control systems, such as implantable cardioversion/defibrillation and pacing, dangerous cardiac rhythms (bardycardia, tachycardia or fibrillation) are commonly detected by measuring the time interval between consecutive cardiac depolarizations. An implantable pacemaker, defibrillator, or external heart monitoring device detects cardiac depolarizations when the sensed cardiac signal exceeds a predetermined amplitude level. The predetermined amplitude level is known as the "sensitivity threshold". The sensing threshold may be fixed, or it may vary over time.
There are extreme variations in cardiac signal amplitude during certain arrhythmias, such as polymorphic tachycardia and fibrillation. Therefore, a fixed sensing threshold may not be appropriate for detecting these arrhythmias. The problem is further complicated when the implantable device delivers pacing pulses to the heart; these pulses cause evoked responses which are quite high in amplitude as compared to normal cardiac depolarizations.
Two prior approaches to compensate for these problems are known. One is to fix the sensing threshold at a value determined by the attending physician after careful study of the variety of amplitudes in cardiac signal activity experienced by a patient; the sensing threshold is programmed into the implantable device. Any cardiac signal larger than the programmed sensing threshold is considered a cardiac depolarization.
The difficulty with this approach is in setting the sensing threshold. If the sensing threshold is too high and the signal amplitude decreases significantly, as is often the case in fibrillation, the device may not sense the arrhythmia. If the sensing threshold is set too low, the device may over-sense. For example, a system designed to detect ventricular depolarizations (R-waves) may erroneously detect atrial depolarizations (P-waves) or ventricular recovery (T-waves). Filtering may help eliminate the erroneous detection of the P-waves and T-waves, but if the pass band of the filter is too narrow, it may also eliminate detection of the fibrillation signals.
A second approach is to set the sensing threshold proportional to the amplitude of a sensed cardiac signal each time a cardiac depolarization is sensed. The sensing threshold is then allowed to decrease over time between consecutively sensed cardiac cycles so that in the event a sensed cardiac signal amplitude decreases significantly, the device is still able to detect the lower level cardiac signals.
The problem with the second approach is that it becomes difficult to adjust the sensing threshold to an appropriate level if the patient requires pacing due to bradycardia. FIG. 1 illustrates the response of such a system that senses R-waves according to this method. The sensing threshold is adjusted to one-half of the R-wave amplitude when an R-wave is sensed. However, the evoked response due to the first pacing pulse causes the sensing threshold to be set so high that a second spontaneous R-wave is not sensed. Because the system did not sense the second spontaneous R-wave, a pacing pulse is delivered to the patient inappropriately.